Simulcast paging systems are well known in the art of paging communication systems. For example, U.S. Pat. No. 5,369,682, assigned to the same assignee as the present invention and incorporated herein by reference, discloses a digital simulcast paging system. In general, such a system includes a paging switch connected to the public switched telephone network, and a plurality of base stations. A caller wishing to page a subscriber of the paging system calls the paging switch using the public switched telephone network (PSTN). The paging switch then formulates a page to the subscriber and distributes the page to each of the paging base stations. The paging base stations then simultaneously broadcast (simulcast) the page. The subscriber receives the page through a personal paging unit (or "pager") that the subscriber carries.
In addition, paging transmitters may include the capability of transmitting pages according to multiple paging protocols. Further, pages with different protocols may be time multiplexed to increase throughput and decrease the system's costs. An example of such a transmitter is disclosed in co-pending and commonly assigned U.S. patent application Ser. No. 08/601,118 entitled "Digital Linear Transmitter Using Predistortion", which is incorporated herein by reference. Thus, for example, a paging transmitter may be capable of transmitting pages according to both POCSAG and FLEX.TM. protocols.
In a simulcast paging system, difficulty can arise in those geographic areas that can receive signals from more than one paging station. These geographic areas are also known as the "overlap regions." In these overlap regions a phase cancellation phenomenon can be observed. The phase cancellation condition occurs as signals from more than one base station are received in an overlap region. Due to the increased accuracy and stability of the present generation of transmitters, the phase cancellation condition has become an issue of even greater concern. More specifically, because these high accuracy transmitters each output almost precisely identical frequencies in the page signals, at certain locations within the overlap regions, the signals are 180.degree. out of phase, thereby canceling each other out. These phase cancellation areas are commonly referred to as "standing null points" within the overlap region.
Some conventional systems attempt to alleviate the above phase cancellation problem by using a fixed frequency offset between adjacent transmitters. Properly chosen, the fixed frequency offset prevents "standing null points" in the overlap regions without degrading the system performance beyond acceptable levels. More specifically, the fixed frequency offset between adjacent transmitters causes the null points within the overlap area to move. The phase cancellation condition is believed to occur periodically at a given location in the simulcast overlap environment at a rate of 1/f.sub.0, where f.sub.0 is the frequency offset (i.e., the frequency difference between the received signals). In general, the duration of the phase cancellation condition at the given location is inversely proportional to the offset frequency f.sub.0. Thus, if a pager is placed in the overlap region, a relatively short duration phase cancellation condition occurs at a rate of 1/f.sub.0. However, because a subscriber may often remain at a certain location for relatively long periods of time, this conventional solution may not be acceptable. For example, if the subscriber remains at a fixed location, a page to this subscriber periodically will suffer significant phase cancellation at the 1/f.sub.0 rate, which could introduce errors in the received page.
Another significant problem with fixed offset schemes is that records of the offsets for each transmitter must be stored and maintained so as to ensure that the offsets for adjacent transmitters are different. As protocols change and equipment is updated, this bookkeeping task adds further complexity and cost to the system.
In addition to fixed offset schemes, there are some conventional offset schemes that introduce a frequency offset with random frequencies. For example, such a scheme is described in U.S. Pat. No. 4,570,265 issued to Thro, Feb. 11, 1986. This type of conventional scheme generally uses a hardware random noise source for generating a noise signal which is amplified and low-passed filtered before application separately or in combination with an information signal to a frequency modulation input lead of a corresponding modulator. Therefore, communications between a central station and mobile receivers located in overlap areas will not be interrupted by the nulls for long periods of time.
Although these conventional schemes provide some benefits, these conventional schemes still have shortcomings. For example, the offset generating hardware used in the above conventional schemes cannot be easily changed once implemented. Thus, in the multiple protocol paging systems, the above conventional offset schemes are not self-configurable to provide optimize frequency offset ranges on a protocol basis. Furthermore, these conventional schemes cannot be readily reconfigured to optimize performance if the protocol(s) are modified. Still further, the hardware used in these conventional systems generally do not allow for specific control of the frequency range, randomness, duration and timing of the randomly changing frequency offset.